This invention relates to a radial tire for a construction vehicle.
Tires applicable to the invention are tires of a size of 18.00-25, wide base tires equivalent thereto of a size of 23.5-25, so-called middle sizes, and gigantic tires, which are used for movable working machinery such as dump trucks, scrapers, front end loaders. The invention is intended to increase the rate of operation of the construction vehicles and to considerably reduce expense of tires which is a high percentage in running cost of the construction vehicles by improving tires used for construction in particularly severe restrictions under extraordinary used conditions of use.
Tires are divided into two main classes along progress in development, tires for on the road and tires for off the road. The former, particularly large tires are represented by those for trucks and buses, while the latter are tires used for the construction vehicles above described and the like.
The tires for trucks and buses are used for running at speeds of the order of from several tens Km/h to one hundred and several tens km/h mainly on paved flat roads. In other words, they are used for high speed running. Therefore, sizes of these tires are 10.00-20 to 11.00-20 in Japan and at the maximum 12.00-20 even in the Western countries although there are some exceptions.
In contrast herewith, tires for construction vehicles and the like are used on rough grounds which are not prepared as roads. In addition to dispersion of fragments or broken pieces of stones and rocks on these grounds, they have large irregularities on their surfaces, so that the tires for construction vehicles are obliged to be used at low speeds. Sizes of these tires range from 18.00-25 to as much as 36.00-51 or 40.00-57. In comparison with such large tires, the tires for trucks and buses may be small type tires.
With tires for trucks and buses running at high speeds; treads of the tires are subjected to repeated forces at high frequencies so that wear-resistance is generally the most important criteria. In contrast herewith, with tires for construction vehicles, they have thick rubbers of treads resulting from the large sizes so that repeated forces acting upon the treads will promote heat generation and heat storage cycles. In other words, the tread rubbers are subjected to repeated strain inputs that generate high temperature heat and the heat is stored due to poor heat transfer of the rubbers. Therefore, there is a risk of high temperature rupture of the rubber due to heat storage.
In view of this, although the allowable continuous maximum speeds of the tires for trucks and buses are 100 km/h according to a standard, those values of tires for construction vehicles are considerably low. For example, allowable maximum speeds of tires for dump trucks are 50 km/h according to JIS (referring now to JATMA YEAR BOOK 1988 in substituting therefor) or 64 km/h according to TRA.
Such limitations of speeds are in connection with the fact that maximum loads of the tires for construction vehicles are larger than those of tires for trucks and buses. For example, maximum loads for both tires of 12.00-24 are as follows at inner pressure of 5 kgf/cm.sup.2.
______________________________________ Tires for trucks and buses 2,865 kgf (dual wheels) 3,005 kgf Tires for construction vehicles 3,460 kgf ______________________________________
As can be seen from these data, the maximum load supporting rate of the tires for the construction vehicles is 1.21-1.15 times higher at the same inner pressure than that of the tires for trucks and buses.
In practice, with tires of 12.00-24 16PR for trucks and buses the maximum load is 3,505 kgf (3,340 kgf for dual wheels) at the normal inner pressure of 6.5 kgf/cm.sup.2. As can be seen from this, the tires for construction vehicles must support the same load with the inner pressure lower than that of the tires for trucks and buses.
The same holds true in the tires of larger sizes than that above described because data are calculated by the same calculation equations.
The differences of the tires for construction vehicles in industrial application as to the standards has been explained. The tires for construction vehicles also exhibit differences in conditions of use. The tires for construction vehicles are always under more overloaded conditions than tires for trucks and buses and are of large type having thick tire members and used at relatively slow speeds. Moreover, they are very often used on rough grounds including frequent irregularities and having stones and rocks dispersed thereon. Therefore, the tires for construction vehicles encounter the following difficulties.
(1) Separation failure resulting from thermal fatigue and thermal rupture owing to high temperatures due to excessive heat generation and heat storage, PA1 (2) Failure due to cut caused by stones and rocks (high temperature in tread will aid the fault), and PA1 (3) Failure at the beads due to excessive strains (resulting from axial compressive loads and bending loads) in bead portions which are tire supporting portions owing to superposition of pulsatile alternate loads caused by riding o stones and rocks and irregularities on roads in addition to large loads.
Among these difficulties, the separation failure of item (1) can be basically solved by utilizing radially arranged carcasses of steel cord plies which have theoretically less heat generation and easy heat dispersion. The failure of item (2) can be basically solved by using thick treads which are also accepted in the standards in addition to utilizing a carcass having a radially arranged steel cord ply. However, the failure of item (3) cannot be basically solved by the above carcasses and there is no solution in standards. Under the present condition that radial tires are essential for solving the problems of the items (1) and (2) although their bead portions are weak points in comparison with bias tires using nylon cords, it is urgently required to prevent failures at bead portions. Such an improvement of radial tire construction for preventing failures at bead portions is most important for applying radial tires to construction vehicles as an industrial utilization. The reason will be clarified by the following explanation.
Different from the trucks and buses, it takes time to exchange tires for construction vehicles. Therefore, exchanging tires are usually intentionally effected in a complete factory maintenance together with adjustment and repairing of other portions of the vehicle at a time of tire exchanging which was previously presumed from wear of the tires.
However, if a failure of the tire (in bead portions in most cases) occurs in an unexpected place at an unexpected time, it is generally difficult to transport the vehicle equipped with the faulty tire to the complete maintenance factory. This occurs because of the heavy weight of the vehicle. Even if it is possible to transport the vehicle to the maintenance factory, such transportation of the vehicle is very expensive and time consuming. In addition, the vehicle must be transported to the factory at a time other than the scheduled time so that the maintenance operation for the other portions of the vehicle could not be effected simultaneously in most cases. Therefore, it requires twice the cost and twice the down time (inoperative time) for the vehicle. Accordingly, the failure of a tire at an unexpected time considerably lowers productivity and increases direct costs. The low productivity and the increased costs are very disadvantageous for a user of the construction vehicle.
As above described, the failure at bead portions is the weakest point for the radial tires using steel cord plies for construction vehicles. Parts of the bead portions in contact with rim flanges with a high surface pressure can be regarded approximately a fixed portions which are firmly urged against the rim flanges. On the other hand, the bead portions are subjected to axial compressive forces in radial directions and bending moment. The failure at the bead portion is mainly caused by the fact that the bead portions radially outwardly of the fixed portions are subjected to the axial compressive forces and the bending moments. First, the bead portions located immediately radially outwardly of the rim flange fall down axially outwardly by the bending moments B.M. Therefore, shearing strains are caused by the axial compressive forces A.C. and the bending moment B.M. between turn-up portions of carcass plies and rubbers and between bead portion reinforcing layers embedded in the bead portions and rubbers thereabout under falling down of the bead portions by the bending moment B.M. Moreover, the shearing strains are added with shearing strains in proportion to the amount of the falling down.
The failure of the bead portion is caused by these shearing strains in the following manner. Cracks occur at ends of cords outside the bead portions among reinforcing cords and progress along directions of the cords. Fragments of rubber and gases accumulate in the cracks to cause the portions including the cracks to swell until the accumulated substances are released therefrom and cords therein are broken to cause failures of the bead portions.
In order to avoid this, steel cord reinforcing layers have been arranged along turn-up portions of the carcass plies and organic fiber layers such as nylon layers have been arranged outwardly of the steel cord reinforcing layers for the purpose of equalizing the difference in rigidity caused by the steel cord reinforcing layers.
However, concentration of shearing strains can not be avoided and failures of beads often occur so long as the steel cords are used. In the most popularly used solution among further improved measures, only organic fiber layers such as nylon are laminated outwardly of the bead portions, for example, six layers. However, this arrangement could not sufficiently restrain the falling down of the bead portions and the six integral organic fiber reinforcing layers have a fairly high rigidity although each has a low rigidity so that strains would be likely to concentrate at some positions. Therefore, cracks are apt to occur particularly in the reinforcing layers relatively prematurely resulting in separations.